Traditionally, detergent powders were produced by spray drying However, the spray drying process is both capital and energy intensive and the products were quite bulky, having a relatively low bulk density.
The desire for powders with higher bulk densities led to the development of processes which employ mainly mixing, without the use of spray drying. These mixing techniques offer great flexibility in producing powders of various different compositions from a single plant, by post-dosing various components after an initial granulation stage. The resultant powders have fairly high bulk densities, which is desirable for some product forms. However, many of these non-spray drying techniques are unsuitable for production of powders over a wide bulk density range and in particular, for the production of lower bulk density powders.
One kind of process, which does not involve spray-drying, and which is capable of producing medium bulk densities between those of spray dried and other non-spray dried powders, involves use of a low shear granulator, usually a fluidised bed apparatus. Although fluidised bed granulation processes per se can give good control of bulk density, there was still a need for greater flexibility and in particular, for producing lower BD powders.
A common fluidised bed process involves contacting a neutralising agent such as an alkaline detergency builder with a liquid acid precursor of an anionic surfactant in the fluidised bed or other low shear apparatus. Such a process is described in WO-A-96/04359. The acid precursor is thereby neutralised to the salt form of the surfactant and the surfactant and other components inside the apparatus form granules.
In order to adapt the low shear granulation process of WO-A-96/04359 to produce lower bulk densities, it was proposed in WO-A-99/00475 to incorporate some inorganic acid together with the liquid acid precursor of the anionic surfactant. A subsidiary advantage of this process is that the powders produced show improved dissolution rates in the wash liquor. However, there is no mention of any effect of the particle size of the neutralizing agent used to neutralise the anionic acid precursor and the inorganic acid.
WO-A-00/37605 discloses a process in which an organic (non-surfactant) acid is used in combination with a carbonated neutralising agent to provide products with bulk densities below about 600 g/l. In the examples, the neutralizing agent, sodium bicarbonate, is of 5–40μm size. However, the total disclosure only mentions high shear mixer/granulators and there is no link made between the particle size of the neutralising agent and the bulk density of the product.
It is an object of the present invention to use a non-surfactant salt formed in situ to yield lower bulk densities and optionally also, improved powder properties, in lower shear mechanical granulation processes.
This object is achieved by forming the salt from a neutralising agent having a smaller particle size.